1 concurrency control iii dead lock time stamp ordering validation scheme

1

Concurrency Control III

Dead Lock

Time Stamp Ordering

Validation Scheme

Database Implementation – Concurrency Control Yan

Huang 2

Learning Objectives

Dealing with Deadlock and Starvation Time Stamp Ordering Technique Validation


Huang 3

Deadlocks Detection

Wait-for graph Prevention

Resource ordering Timeout Wait-die Wound-wait


Huang 4

Deadlock Detection

Build Wait-For graph Use lock table structures Build incrementally or periodically When cycle found, rollback victim

T1

T3

T2

T6

T5

T4T7


Huang 5

Resource Ordering

Order all elements A1, A2, …, An

A transaction T can lock Ai after Aj only if i > j

Problem : Ordered lock requests not realistic in most cases


Huang 6

Timeout

If transaction waits more than L sec., roll it back!

Simple scheme Hard to select L


Huang 7

Wait-die Transactions are given a timestamp when they

arrive …. ts(Ti) Ti can only wait for Tj if ts(Ti)< ts(Tj)

...else die


Huang 8

T1

(ts =10)

T2

(ts =20)

T3

(ts =25)

wait

wait

Example:

wait?

Very high level: only older ones have the privilege to wait, younger ones die if they attempt to wait for older ones


Huang 9

Wound-wait Transactions are given a timestamp when they

arrive … ts(Ti) Ti wounds Tj if ts(Ti)< ts(Tj)

else Ti waits

“Wound”: Tj rolls back and gives lock to Ti


Huang 10

T1

(ts =25)

T2

(ts =20)

T3

(ts =10)

wait

wait

Example:

wait

Very high level: younger ones wait; older ones kill (wound) younger ones who hold needed locks


Huang 11

Who die? Looks like it is always the younger ones

either die automatically or killed

What is the reason? Will the younger ones starve?

Suggestions?


Huang 12

Timestamp Ordering Key idea:

Transactions access variables according to an order decided by their time stamps when they enter the system

No cycles are possible in the precedence graph


Huang 13

Timestamp System time when transactions starts An increasing unique number given to each stransaction

Denoted by ts(Ti)


Huang 14

The way it works Two time stamps associated with each variable x

RS(x): the largest time stamp of the transactions read it WS(x): the largest time stamp of the transactions write it

Protocol: ri(x) is allowed if ts(Ti) >= WS(x) wi(x) is allowed if ts(Ti) >=WS(x) and ts(Ti) >=RS(x) Disallowed ri(x) or wi(x) will kill Ti, Ti will restart


Huang 15

ExampleAssuming: ts(T1) = 100, ts(T2) = 200, ts(T3) = 300

T1 T2 T3

R(x);

W(y);

R (y);

W(z);

R(x);

W(z);

R(y);

W(x);

x y z

RS=-1 RS=-1 RS=-1

WS=-1 WS=-1 WS=-1


Huang 16


T1 T2 T3

R(x);

W(y);W(y);

R (y);R (y);

W(z);W(z);

R(x); R(x);

W(z);W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=100 RS=-1 RS=-1

WS=-1 WS=-1 WS=-1


Huang 17


T1 T2 T3

R(x);

W(y);

R (y);R (y);

W(z);W(z);

R(x); R(x);

W(z);W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=100 RS=-1 RS=-1

WS=-1 WS=100 WS=-1


Huang 18


T1 T2 T3

R(x);

W(y);

R (y);

W(z);W(z);

R(x); R(x);

W(z);W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=100 RS=200 RS=-1

WS=-1 WS=100 WS=-1


Huang 19


T1 T2 T3

R(x);

W(y);

R (y);

W(z);

R(x); R(x);

W(z);W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=100 RS=200 RS=-1

WS=-1 WS=100 WS=300


Huang 20


T1 T2 T3

R(x);

W(y);

R (y);

W(z);

R(x);

W(z);W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=200 RS=200 RS=-1

WS=-1 WS=100 WS=300


Huang 21


T1 T2 T3

R(x);

W(y);

R (y);

W(z);

R(x);

W(z);

R(y); R(y);

W(x);W(x);

x y z

RS=200 RS=200 RS=-1

WS=-1 WS=100 WS=300

T1 is rolled back


Huang 22

Net result of TO scheduling Conflict pairs of actions are taken in the order of their

home transactions But the basic TO does not guarantee recoverability


Huang 23

Validation

An optimistic scheme

Transactions have 3 phases:

(1) Read all DB values read writes to temporary storage no locking

(2) Validate check if schedule so far is serializable

(3) Write if validate ok, write to DB


Huang 24

Time stamps of a transaction Ti Start(Ti) Validation(Ti) Finish(Ti)


Huang 25

Key idea Make validation atomic If T1, T2, T3, … is validation order, then resulting

schedule will be conflict equivalent to Ss = T1 T2

T3...


Huang 26

Schedule

T1 T2

Read(A)

A A+100;

Read(A)

A Ax2;

Read(B);B B+100

validate

Write(A)

Write(B);

Read(B)

B Bx2;

validate

Write(A)

Write(B);


Huang 27

Example of what validation must prevent:

RS(T2)={B} RS(T3)={A,B}

WS(T2)={B,D} WS(T3)={C}

time

T2

start

T2

validate

T3

validateT3

start

=

T2

finishes

T3

finishes


Huang 28

T2

finishphase 3

Example of what validation must prevent:

RS(T2)={B} RS(T3)={A,B}

WS(T2)={B,D} WS(T3)={C}

time

T2

start

T2

validated

T3

validatedT3

start

=

allow

T3

start


Huang 29

Another thing validation must prevent:RS(T2)={A} RS(T3)={A,B}

WS(T2)={D,E} WS(T3)={C,D}

time

T2

validatedT3

validated

finish

T2BAD: w3(D) w2(D)


Huang 30

finish

T2

Another thing validation must prevent:RS(T2)={A} RS(T3)={A,B}

WS(T2)={D,E} WS(T3)={C,D}

time

T2

validatedT3

validated

allow

finish

T2


Huang 31

Validation Rule When start validating T

Check RS(T) WS(U) is empty for any U that started but (did not finish validation before T started)

Check WS(T) WS(U) is empty for any U that started but (did not finish validation before T started validation)


Huang 32

Exercise:

T: RS(T)={A,B} WS(T)={A,C}

V: RS(V)={B} WS(V)={D,E}

U: RS(U)={B} WS(U)={D}

W: RS(W)={A,D} WS(W)={A,C}

startvalidatefinish


Huang 33

Exercise:





startvalidatefinish


Huang 34

Exercise:





startvalidatefinish


Huang 35

Exercise:





startvalidatefinish

W is rolled bak

1 concurrency control iii dead lock time stamp ordering validation scheme

Documents

tsti slide

older ones slide

t i wounds t j

locks slide

wait example

transaction t

t j rolls

wound younger ones